A drill chuck such as described in German patent documents 3,416,986 and 3,426,808 for use with a tool having a shaft typically has a chuck body centered on and rotatable about an axis and an adjustment ring centered on the axis, rotatable about the axis on the chuck body, and axially fixed on the chuck body. The chuck body is formed with a plurality of angularly equispaced jaw-guide passages centered on respective jaw axes all lying on the surface of an imaginary cone centered on the axis. The adjustment ring is formed with a screwthread centered on the axis and exposed in the guide passages. Respective jaws displaceable in the passages along the respective axes are each formed with a row of teeth meshing with the screwthread so that relative rotation of the ring and chuck body in one direction moves the jaws radially inward and axially forward and opposite relative rotation moves the jaws radially outward and axially rearward.
So that such a chuck can be locked, the chuck body is provided with a spring-biased latch element to engage with any of an annular array of recesses formed on the adjustment ring. This element can move axially but not angularly on the chuck body and serves to keep the chuck in the position it is locked in. When the adjustment ring is forcibly rotated, the locking element ratchets or slides over the recesses in this ring, permitting the chuck to be tightened and loosened.
While such a system is effective in preventing the chuck from loosening, it is nonetheless possible, in particular when hammer-drilling, for the chuck jaws to bite into the drillbit shaft. In this case, even though the chuck body and adjustment ring have not moved relative to each other, the chuck has in effect loosened its grip on the tool.
Another disadvantage of the known system is that it is necessary to use considerable force at all times to rotate the adjustment ring on the chuck body, so as to overcome the spring of the latch element and cause it to slip. This is disadvantageous and serves no useful function.